High frequency core material and core and process for making said material



Patented Jan. 11, 1944 2,339,137 HIGH FREQUENCY CORE MATERIAL AND CORE AND PROCESS FOR- MATERIAL MAKING SAID GodshallgBerge, Chicago, 111., assignor to Johnson Laboratories, Inc., Chicago, 111., a. corporation of Illinois No Drawing. Application June 21, 1940, Serial No. 341,734

8 Claims.

the"{'coil- =.on"-whos hnergyithey feed and whose eiliciency they decrease-fa phenomenon that becomes increasingly" critical .with, increasing frequency' until the currents in the coil cease to flow. unless proper precautions are taken.

I When magnetic materials are subdivided their initial permeabilitles are considerably reduced.

- In general therefore, the permeability of cores suitable for high frequency operation is relatively low as compared with the permeabilities of the cores of the low frequency and heavy current art where precautions as to the prevention of eddy current losses need not be nearly as exacting as in the high frequency art.

Whether a high frequency core is used in a fixed core inductor or whether it is used as the tuning element of a movable core variometer, its permeability may be regarded as a direct measure of its utility, provided its conductivity and hence its losses have been kept low, and especially in the case of core variometers a high permeability is desirable because it is the permeability of the tuning core that primarily determines the range of inductance variation obtalnable.

The demand for high frequency cores of higher permeabilities than so far obtainable, has recently become more urgent than ever, especially since the art has turned to core-tuned loop antenna circuits. When such a circuit is tuned by a movable core variometer, the loop which forms part of the total inductance of the circuit but which is removed from the effect of the tuning core, materially reduces the range of inductance variations obtainable in said circuit as compared with the range normally expected from the permeability of the tuning core, yet if the circuit is to be tuned over a required range, it is essential to have a tuning core whose effective permeability is so large that it covers this tuning range in spite of the coverage reducing effect of the loop.

It is an object of the present invention to provide a magnetic core material of high permeability that can readily be reduced to such fine degrees of subdivision as are necessary for high frequency operation where particle sizes down to 1 micron and below are required.

It is a furthenobject of the present invention to provide a magnetic material which when made into cores of the type referred to yields a higher permeability than heretofore obtainable without harmful increase in the eddy current losses. More specifically it is an object of the present invention to provide a core material that yields higher permeability without increase in electric conductivity, thus imparting to coils with which it is used a better ratio of useful inductance to resistance in the broadcast range than has been possible with any prior known. material.

Another defect of prior known cores has been that they deteriorated in time due to corrosion produced by atmospheric influences which resulted in harmful changes of their permeabilities.

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It is another object of the present invention to provide a core material which has considerable resistance to atmospheric influences of any kind and which retains its permeability unimpaired over practically unlimited periods of time.

The present invention is based upon the discovery that iron and tin are capable of forming materials for the construction of magnetic cores for high frequency purposes which have the desirable qualities mentioned above. The materials of the present invention consist of from 98-80% of iron and from 2-20% of tin by weight.

Compressed powder cores made from the new materials have a higher permeability than obtainable with previously known magnetic materials and hence when used in high frequency variometers of the movable core type, they yield larger ranges of inductance variations than heretofore believed possible with devices of this type. Furthermore, core materials made from the new magnetic material exhibit greater resistivity to the passage of electrical currents than core materialsmade from most of the prior known magnetic materials of metallic character and hence they yield-inductors of a highly favorable ratio of inductance to resistance over the broadcast range.

Moreover, when the new material of the present invention is in the powder condition in which it is used for the production of cores, it exhibits self-lubricating characteristics in that it can readily be compressed to higher densities than formerly obtainable, without the application of such large pressures as reduce the resistivity of the resultant cores below the limit permissible for high frequency operation. This permits of obtaining an additional increase in permeability and it also means less wear for the compression molds than has previously been experienced.

Cores made'from the new material show no a g effects, even under unfavorable atmospheric conditions. They retain their desirable properties for practically unlimited periods of time.

The new magnetic material may be produced from metallic iron and tin or from any of their oxides as initial materials.

For instance, pure iron oxide F620: and tin oxide SI1O2 may be taken and reduced to powder by any suitable method. In order to facilitate the comminution of the iron oxide the same may for a short time be subjected to a. temperature of 2000 F. which renders it brittle. The finer the powder used as initial material the more satisfactory the process will operate and the better will be the resultant product. Generally powders of such a fineness as to pass a 200-mesh screen will suflice.

The relative amounts of iron oxide and tin oxide depend upon the nature of the product which it is intended to produce. Thus, if the final prodnot is to consist of 94% iron and 6% tin the quantitles of oxides mixed together must be so related that the Weights of elementary iron and tin actually contained in the ingredients are related as 94:6. For instance, iron oxide F8203 has a molecular weight of 159.68, the weight of the iron contained in the compound being 111.68, and tin oxide SnOz has a molecular weight of 150.7, the weight of the tin being 118.7. Consequently 134.38 lbs. of iron oxide powder (FezOa) mixed with 7.62 lbs. of tin oxide powder (SnOz) yield 100 lbs. of a product having a tin content of 6% by weight.

-The two ingredients are carefully blended and the mixture is then subjected to a temperature of at least 1100 F. but preferably from 1400" F. to 1500 F. in a hydrogen furnace until the oxides are properly reduced and a tin-iron alloy is formed. The period of treatment depends upon the temperatures employed and on the quantities treated. The lower the temperature and the larger the quantity treated the longer the process will take and vice versa. The time when the oxides have been completely reduced to metallic condition and the desired iron-tin alloy has been formed can readily be determined by taking samples from the furnace.

In the event of one or both of the initial in gredients being oxides, as is the case in the specific example I am about to describe, the abovementioned heat treatment has to be performed in an atmosphere of hydrogen or any other atmosphere that is capable of reducing metallic oxides, such as methane, butanefetcq while in case that both the initial ingredients are pure metals atmospheres of inert gases such as 8. nitrogen may be employed. I prefer, however, to use hydrogen or other reducing gases in all instances because such atmospheres will purify initial ingredients of supposedly pure metallic character from any oxides that may have formed on their surfaces.

The reduction process may immediately be followed by an annealing process in that the temperature in the furnace and not to exceed about 1550 F. is progressively decreased to about 400 F. over a period of at least four hours, whereupon the mass is gradually cooled to room temperature in an atmosphere of dry hydrogen. As an alternative the mass may be cooled down to room temperature immediately upon completion of the reduction process, whereupon it may be ground to powder and subjected to a separate annealing process under approximately the same temperatures and for about the same period of time as the annealing process described above.

After cooling, the annealed product is comminuted by any suitable method such as ball mill ing, and graded by an air separator according to the particular requirements made for the fineness of the intended final product.

- be compressed into solid bodies.

For comminuting the mass delivered by the reduction and annealing processes I prefer to use a brush-like instrument which is provided with a multitude of steel pins having fine needle points. I press this instrument through the lumps and conglomerates of crystals that have formed, in order to break them up. The more often this method is repeated the finer will be the resultant powder. For better results I repeat the process with various brushes, each successive one having an increasing number of pins of increasing fineness per unit area. Alternatively the instrument may be such that the number of pins per unit area increases over its operating surface with the needle points getting finer and finer the denser they stand, the arrangement being such that the lumpy mass coming from the furnace is first led to places having fewer needles and is slowly moved towards the areas having an increasing density thereof.

The powder is then again subjected to an annealing process in an inert or a reducing atmosphere. I prefer to use hydrogen. First a temperature of about 1500 F. is employed which is gradually lowered to 1100 F. over a period of at least two hours whereupon it is further lowered to 400 F. over a period of at least three hours. Finally the mass is cooled down to room temperature. The exact duration of the periods of treatment indicated above depend, of course, upon the quantities treated, but I have found that for best results the second stage of the process during which temperatures from 1100 F. to 400 F. are employed should last materially longer than the first.

The material may now be subjected to a short grinding process for which the above described brush milling method may be employed, to make sure that lumps that may have formed during the annealing process are broken up and that the whole mass is in the powder form necessary for the manufacture of high frequency cores.

The powder may then be insulated in any manner suitable for high frequency cores such as, for instance, with a varnish made with Chinawood oil, and the insulated powder may then be compressed in molds with a suitable binder such as Bakelite dissolved in alcohol. The total amount of insulator and binder added to the powder should preferably not exceed 4% of the weight of the finished core. The mas may then In connection herewith it will be found that relatively low pressures such as from 25 to 35 tons per square inch will suifice to compress the charge into cores of considerable specific densities up to 6 and even more, which are contributory towards high permeabilities.

A core material made from the powder provided by the process described above and which consisted of 6% of tin and 94% of iron by weight showed an apparent permeability of 18.62 (measured on a ring core having an inner diameter of 1'', an outer diameter of 1 /4" and a height of 1%"). Its specific resistance was 2.'l4 10 ohms per cm. and its specific density was 5.8. A core of 1%" length and 0.2" diameter when cooperating with a solenoid coil of the type used for movable core variometers adapted to operate in the broadcast range showed an effective permeability of 12.2 which was suflicient to tune a circuit having a capacity of 153.6 F over a frequency range from 1560 kilocycles to 446 kilocycles. The coil had an air core inductance of 68.7 ,uh and was wound with No. 38 P. E. wire on a Bakelite tube having an outside diameter of 0.221" over a length of 11%" with 220 turns per inch.

The Q value (wL/R) of the circuit at the upper end of the frequency range where the core was completely withdrawn from the coil was 41. In spite of the considerable permeability of the core the Q value of the circuit at the lowest frequency where the core was completely inserted into'the coil was very favorable, namely, 94.

The new material may likewise b used with excellent results for cores in the so-called fixedcore inductors of the high frequency art.

It has been found that while core materials having the desired properties mentioned above can be produced from compositions ranging in tin content from 220%. a. composition of approximately 6% of tin and approximately 94% of iron yields highest permeability, combined with very satisfactory Q values in the broadcast range.

Cores made from the new material of the present invention were subjected to weather tests at a temperature of 140 F. and at a relative humidity of 94% over a period of 100 hours without any noticeable change of their magnetic and electrical properties. Not even their external appearance showed any considerable change.

While I have illustrated my invention with the aid of particular examples of the product and the processes for producing same, it will be understood that I do not limit myself thereto but that I may employ equivalents without departing from the scope of the appended claims.

Having thus described my invention What I claim is:

1. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material produced in a single treatment by the reduction of a mixture of tin oxide and iron oxide in a finely divided state at a temperature of from 1500 F. to 1400 F., thereby forming an alloy of iron and tin, the tin content ranging from 2 to by weight and the iron content ranging from 98 to 80% by weight.

2. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material comprising a comminuted alloy of tin and iron containing by weight 2% to 20% of tin and. 98% to 80% of iron, which alloy particles have been subsequently annealed in two steps of subjecting the material to a temperature of not over 1550 F. and gradually cooling to a temperature of about400 F.

over a period of at least four hours and of further subjecting the material to a temperature of about 1500" F. and gradually cooling. to about 1100 F. over a period of at least two hours, with continued gradual cooling to a temperature of about 400 F. over a period of at least three hours.

3. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material produced in a single treatment by the reduction of a mixture of tin oxide and iron oxide in a finely divided state, thereby formc an alloy of iron and tin, the tin content rang- .ro'm 2- to 20% weight and the iron content 1g from 96 to 80% by weight, the resultant i being subsequently annealed in two oi subjecting the material to temperature over i550 F. and gradually cooling to a temperature of about 400 over a period of at least four hours and of further subjecting thmaterlal to a temperature of about 1500 F. 8.114 gradually cooling to about 1100 F. over a periol of at least two hours, with continued gradua cooling to a temperature of about 400 F. ove

a period of at least three hours.

4. A ferromagnetic core for use with high fre quency inductors, constructed of powdered mag netic material produced in a single treatment b; the reduction of a mixture of tin oxide and H01 oxide in a finely divided state at a temperatur of from 1500 F. to 1400 F., thereby forming a1 alloy of iron and tin, the tin content being sub stantially 6% and the iron content being sub stantially 94% by weight 5. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material comprising a comminuted allo: of tin and iron containing by weight substantiall: 6% of tin and substantially 94% of iron, whicl alloy particles have been subsequently annealec in two steps of subjecting the material to a temperature of not over 1550 F. and gradually cooling to a temperature of about 400 F. over 2 period of at least four hours and of further subjecting the material to a temperature of abow 1500 F. and gradually cooling to about 1100 F over a period of at least two hours, with continued gradual cooling to a temperature of abou1 400 F. over a period of at least three hours.

6. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material produced in a single treatment by the reduction of a mixture of tin oxide and iror oxide in a finely divided state, thereby forming an alloy of iron and tin, the tin content being substantially 6% and the iron content being substantially 94% by weight, the resultant material being subsequently annealed in two steps of subjecting the material to a temperature of not over 1550" F. and gradually cooling to a temperature of about 400 F. over a period of at least four hours and of further subjecting the material to a temperature of about 1500 F. and gradually cooling to about 1100 F. over a period of at least two hours, with continued gradual cooling to a temperature of about 400 F. over a period of at least three hours.

7. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material produced by the reduction of a mixture of tin oxide and iron oxide in a finely divided state at a temperature in the range oi from substantially 1500 F. to at least 1100- F., and at the same time forming an alloy of the iron and tin so produced, the tin content of the product averaging from 2 to 20% by weight and the iron content of the product averaging from 98 to by weight.

8. A ferromagnetic core for use with high frequency inductors, constructed of powdered magnetic material produced by the reduction of a mixture of tin oxide and iron ovide in a finely divided state at a temperature in the range of from substantially 1500 F. to at least 1100 F., and at the same time forming an alloy of the iron and tin so produced, the tin content of the product averaging substantially 6% and the iron content of the product averaging substantially 94% by weight.

GODBHALK BERGE. 

